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USGS1998

Contributions to the gold metallogeny of northern Nevada

Nevada is one of the Earth's premier gold producing regions, accounting for approximately 64 percent of the U.S and nine percent of the world total. The impact of these mines on nearby local economies and on our national balance of payments is profound, and will continue well into the next century. Of principal importance in this region are giant sedimentary-rock-hosted (Carlin-type) deposits. These are some of the world's largest deposits, but yet are poorly understood. Other sedimentary-rock hosted deposits in the region, the distal-disseminated Ag-Au type, are genetically related to shallow plutonic complexes. Hot-spring gold-silver systems associated with Tertiary volcanic rocks represent a third type of precious metal deposit in northern Nevada. These deposits, despite being generally smaller than sedimentary-rock-hosted gold deposits, are also important gold-silver resources. Aspects about the geologic and metallogenic setting of gold-silver deposits in northern Nevada are addressed in the twenty-two chapters that compose this volume. The volume is organized along four themes: (1) crustal structure; (2) Carlin-type deposits; (3) pluton-related gold-silver deposits near Battle Mountain; and (4) hot-spring gold-silver deposits. This Open-File Report, the result of ongoing geologic and mineral-resource investigations, provides a basis for mineral exploration, for land-use planning decisions, and for environmental questions in northern Nevada.

Open-File Report↗

USGS1998

Contributions to the gold metallogeny of northern Nevada

Open-File Report↗

USGS1989

Soil geochemistry of Mother Lode-type gold deposits in the Hodson mining district, central California, U.S.A.

The Hodson mining district is in the westernmost foothills of the Sierra Nevada in California, about 17 km west of the town of Angels Camp. This district is part of the West Gold Belt, which lies about 12-16 km west of, and generally parallel to, the better known Mother Lode Gold Belt in central California. The district produced several million dollars worth of Au between about 1890 and 1940. The geologic setting and mineral deposits in the West Gold Belt are generally similar to those in the Mother Lode Gold Belt. Rocks in the study area are of Jurassic age and consist of a mixture of (1) fine-grained, generally thin-bedded, clastic sedimentary rocks that have been metamorphosed to slates, schists, and phyllites, and (2) massive volcanic flows and welded tuffs that have been metamorphosed to metabasalts and metatuffs. All rocks were intensely faulted and folded during the Late Cretaceous Nevadan orogeny; northnorthwest- and northwest-trending faults dominate. Mining in the area was of low-grade gold-pyrite ores occurring principally in the carbonatized wall rocks adjacent to the major northwest-trending Hodson fault and its splays. Minor amounts of other sulfide minerals (principally chalcopyrite, arsenopyrite, sphalerite, and galena) are locally associated with the Au deposits.

California↗

USGS2009

Carbonatites of the world, explored deposits of Nb and REE— Database and grade and tonnage models

This report is based on published tonnage and grade data on 58 Nb- and rare-earth-element (REE)-bearing carbonatite deposits that are mostly well explored and are partially mined or contain resources of these elements. The deposits represent only a part of the known 527 carbonatites around the world, but they are characterized by reliable quantitative data on ore tonnages and grades of niobium and REE. Grade and tonnage models are an important component of mineral resource assessments. Carbonatites present one of the main natural sources of niobium and rare-earth elements, the economic importance of which grows consistently. A purpose of this report is to update earlier publications. New information about known deposits, as well as data on new deposits published during the last decade, are incorporated in the present paper. The compiled database (appendix 1; linked to right) contains 60 explored Nb- and REE-bearing carbonatite deposits - resources of 55 of these deposits are taken from publications. In the present updated grade-tonnage model we have added 24 deposits comparing with the previous model of Singer (1998). Resources of most deposits are residuum ores in the upper part of carbonatite bodies. Mineral-deposit models are important in exploration planning and quantitative resource assessments for two reasons: (1) grades and tonnages among deposit types vary significantly, and (2) deposits of different types are present in distinct geologic settings that can be identified from geologic maps. Mineral-deposit models combine the diverse geoscience information on geology, mineral occurrences, geophysics, and geochemistry used in resource assessments and mineral exploration. Globally based deposit models allow recognition of important features and demonstrate how common different features are. Well-designed deposit models allow geologists to deduce possible mineral-deposit types in a given geologic environment, and the grade and tonnage models allow economists to estimate the possible economic viability of these resources. Thus, mineral-deposit models play a central role in presenting geoscience information in a useful form to policy makers. The foundation of mineral-deposit models is information about known deposits. This publication presents the latest geologic information and newly developed grade and tonnage models for Nb- and REE-carbonatite deposits in digital form. The publication contains computer files with information on deposits from around the world. It also contains a text file allowing locations of all deposits to be plotted in geographic information system (GIS) programs. The data are presented in FileMaker Pro as well as in .xls and text files to make the information available to a broadly based audience. The value of this information and any derived analyses depends critically on the consistent manner of data gathering. For this reason, we first discuss the rules used in this compilation. Next, the fields of the database are explained. Finally, we provide new grade and tonnage models and analysis of the information in the file.

Open-File Report↗

USGS1972

Appraisal of iron deposits in southern and western Turkey

Between May 20 and June 17, 1969, previously known iron deposits were examined widely at eight separate localities in western Turkey. The object of the examinations was to learn the, nature, geologic setting, and approximate size of each deposit, to review prior estimates of size, and possibly recommend additional exploratory work.. The full extent of each deposit is poorly known at the present time, so recommended additional work entails drilling, digging trenches or pits, geologic mapping or, combinations of these activities. On Qaldagi Mountain an area of about 1 sq km is capped by bredciated chert under which may be a continuous zone of mixed iron oxides and chert fragments. The thickness of the ferruginous zone is poorly known but is as much as 12 meters, in at least one place. The- ferruginous material and chert appear to have formed by the weathering of serpentine, bun this concept needs further testing. Drilling is recommended to determine the grade, thickness, and extent-of the ferruginous zone beneath the cherty cap. Inasmuch as mining by hand sorting is in progress, part of the deposit can be considered to be marginally in the category of iron reserves. The Keceborlu iron deposit consists of earthy to slightly compacted hematite and limonite mixed with small chert fragments. The surface area underlain by ferruginous rock is about 5,000 to 7,500 sq meters. The maximum known thickness of the deposit is about 7 meters. Iron appears to have been concentrated by weathering and oxidation of cherty limestone. The deposit is probably either a remnant of a once more extensive weathered cap, or a sink hole filling. The Keceborlu area warrants a low priority for further exploration, but one drill hole is recommended to test the thickness of the deposit. The iron deposits at Mellec are layered and vein-magnetite replacements of limestone. The six known deposits are discontinuous. No additional. work is recommended. ' The Gilindire Iron deposit consists of irregular concentrations of pisolitic and earthy hematite and 'limonite along an unconformity or disconformity between two groups of limestone. The ferruginous zone is incompletely known around the rim of the large Gilindire syncline. Data from trenches 5 to 6 km around the syncline--about ? the possible length of the ferruginous zone--provide the main knowledge about the size and grade of ferruginous lenses. The ferruginous lenses range in thickness from a fraction of a meter, to about 3 meters, but appear to average 1 meter or less, and range in grade from about 10 to 37 percent iron. No additional exploration work is recommended at Gilindire. The Buyukeceli deposit consists of veinlike masses of earthy and compact hematite and limonite cutting fresh limestone. The veins apparently originally contained siderite which has been weathered and converted to iron oxide. Further exploration by drilling is recommended at such time as other largest deposits are able to be brought into the development stage in the Mediterranean coastal area of Turkey. The iron deposits overlooking Bayas on the Gulf of Iskenderun ere in one or more layers along the west-facing front of the Amanus Mountain Range, between beds of gently to moderately east-dipping limestone. Isolated exposures may represent a once-continuous ferruginous bed that has been blockfaulted and intruded by serpentine. The ferruginous bed (or beds) is 20-30 meters thick, and consists of a mixture of very fine grained hematite and claylike material. Iron content ranges from 20 to 40 percent and aluminum, averages about 15 percent. Available data on distribution are scant but suggest that one ferruginous bed may be 1-2 kilometers long, 500 meters wide and 20 meters thick. The potentially large size of the Payas deposits warrants an early coordinated program of drilling end beneficiation testing. An iron deposit was examined on a conspicuous limestone ridge in the Syrian graben east of the Amanus Moun

Open-File Report↗

USGS1979

Geology, petrology, and chemistry of the Leadville Dolomite: host for uranium at the Pitch Mine, Saguache County, Colorado

Newly documented uranium ore in the Pitch Mine occurs chiefly in brecciated Mississippian Leadville Dolomite along the Chester reverse fault zone, and to a lesser extent in sandstone, siltstone, and carbonaceous shale of the Pennsylvanian Belden Formation and in Precambrian granitic rocks and schist. Uranium-mineralized zones are generally thicker, more consistent, and of higher grade in dolomite than in other hosts, and roughly 50 percent of the new reserves are in dolomite. Strong physical control by dolomite is evident, as this is the only lithology that is pervasively brecciated within the fault slices that make up the footwall of the reverse fault zone. Other lithologies tend to either remain unbroken or undergo ductile deformation. Chemical controls are subtle and appear to involve chiefly formation of FeS2 as pyrite and marcasite, which accompany uranium. Leadville Dolomite in the area is about 130 m thick and is predominantly nonfossiliferous dolomicrite. In the Pitch Mine, Leadville Dolomite is bound by faults and maximum known thickness is about 17 m. Mud texture, paucity of fossils and other allochems, thin laminations, and probable algal mat structures suggest sedimentation in a tidal-flat (possibly supratidal) environment. Preservation of mud texture and lack of replacement features indicate that dolomitization was an early, prelithification process, as in modern tidal flats, and produced a chemically and texturally uniform rock over tens of meters with relatively few limestone beds surviving. The sedimentary and diagenetic environment of the tidal-flat dolomite, apparently most favorable for uranium deposits, probably obtained over a large area and should consistute an exploration target over a broad area of central Colorado. Carbonate rocks of the Belden Formation, in contrast to those of the Leadville, contain calcite in great excess of dolomite, more than 5 percent silt-size quartz and clay, and abundant fossils and oolites. Belden limestones (sandy micrite and sandy wackestone) probably were deposited in an intertidal or subtidal environment. Very little uranium ore occurs in these rocks. Chemical aspects, such as the iron, sulfur, and organic carbon contents, are very similar to those of Leadville dolomites, and hence seem favorable, but Belden limestones generally are only mildly fractured. The minor-element content of ore-bearing dolomites is generally normal judging from the relatively scarce data yet published for comparable rocks. Elements enriched in ore include iron, sulfur, molybdenum, and lead. One surface expression of ore in dolomite is ocher-colored, leached, porous gossan that is characterized by residual silica and limonite and by high radioactivity but low chemical uranium content.

Open-File Report↗

USGS1925

Power resources of Snake River between Huntington, Oregon and Lewiston, Idaho: Chapter C in Contributions to the hydrology of the United States, 1923-1924

Thousands of people are familiar with that part of Snake River where it flows for more than 300 miles in a general westward course across the plains of southern Idaho, but few have traversed the river where it flows northward and for 200 miles forms the boundary between Idaho and Oregon and for 30 miles the boundary between Idaho and Washington. Below the mining town of Homestead, Oreg., which is the end of a branch line of the Oregon Short Line Railroad, Snake River finds its way through the mountain ranges that seem to block its way to Columbia River in a canyon which, though not so well known, so majestic, nor so kaleidoscopic in color, is in some respects worthy of comparison with the Grand Canyon of the Colorado, for at some places it is deeper and narrower than the Grand Canyon at El Tovar. The Snake, unlike the Colorado, can be reached at many points through the valleys of tributary streams, and the early prospectors no doubt thoroughly explored all parts of the canyon. To traverse the river between Homestead, Oreg., and Lewiston, Idaho, is, however, a difficult undertaking and there are only a few records of boat journeys through the entire stretch. It has long been known that this portion of Snake River contains large potential water powers, but until recently no detailed surveys or examinations covering the entire stretch of the river had been made to determine their location or extent. A railroad has been proposed between Homestead and Lewiston which would provide a direct connection between the railroad systems of northern and southern Idaho. One function of the Geological Survey is to determine the possible interface between transportation routes on land and potential water-power development, and the information set forth in this paper has a bearing on that problem.

Idaho;Oregon↗

USGS1992

The Alaska Mineral Resource Assessment Program: Background information to accompany geologic and mineral-resource maps of the Cordova and Middleton Island quadrangles, southern Alaska

This report summarizes recent results of integrated geological, geochemical, and geophysical field and laboratory studies conducted by the U.S. Geological Survey in the Cordova and Middleton Island 1°x3° quadrangles of coastal southern Alaska. Published open-file reports and maps accompanied by descriptive and interpretative texts, tables, diagrams, and pertinent references provide background information for a mineral-resource assessment of the two quadrangles. Mines in the Cordova and Middleton Island quadrangles produced copper and byproduct gold and silver in the first three decades of the 20th century. The quadrangles may contain potentially significant undiscovered resources of precious and base metals (gold, silver, copper, zinc, and lead) in veins and massive sulfide deposits hosted by Cretaceous and Paleogene sedimentary and volcanic rocks. Resources of manganese also may be present in the Paleogene rocks; uranium resources may be present in Eocene granitic rocks; and placer gold may be present in beach sands near the mouth of the Copper River, in alluvial sands within the canyons of the Copper River, and in smaller alluvial deposits underlain by rocks of the Valdez Group. Significant coal resources are present in the Bering River area, but difficult access and structural complexities have discouraged development. Investigation of numerous oil and gas seeps near Katalla in the eastern part of the area led to the discovery of a small, shallow field from which oil was produced between 1902 and 1933. The field has been inactive since, and subsequent exploration and drilling onshore near Katalla in the 1960's and offshore near Middleton Island on the outer continental shelf in the 1970's and 1980's was not successful.

Alaska↗

USGS1953

Carnotite resources of the Dolores bench, Montrose County, Colorado

The Dolores bench is about 2 miles northwest of Uravan, Montrose County, Colo. From 1913 to November 1952 about 95,000 short tons of ore averaging 0.40 percent U 3 O 8 and 2.0 percent V 2 O 5 was mined from the Dolores bench. The production represents three periods of activity--1913-18, 1938-43, and 1948-52. The ore deposits are in broad sandstone lenses near the top of the Salt Wash member of the Jurassic Morrison formation. The deposits are mainly impregnations of sandstone by carnotite and vanadiferous clay minerals. The deposits are irregular tabular layers which occasionally include pod-like masses called “rolls”. The rolls as well as the mineralized areas enclosing them have a poorly defined northeast trend. Between December 4, 1951, and August 1, 1952, the U.S. Geological Survey diamond-drilled 183 holes totaling 53,654 feet. The indicated and inferred reserves of carnotite-bearing material, and the pounds of contained U 3 O 8 and V 2 O 5 area summarized in table 1. At the highest thickness and grade cutoffs (1 foot thick and 0.10 percent U 3 O 8 or 1.0 percent V 2 O 5 ), indicated and inferred ore reserves total 90,000 short tons, averaging 0.33 percent U 3 O 8 and 2.11 percent V 2 O 5 . These reserves include only those discovered by U.S. Geological Survey drilling. Potential reserves, whose existence is based on geologic evidence alone, are estimated to be about 10,000 short tons, averaging about 0.30 percent U 3 O 8 and 2.00 percent V 2 O 5 . No additional exploration of the Dolores bench is planned by the Geological Survey. Diamond drilling by claim owners is recommended in several parts of the area.

Colorado↗

USGS2022

Documenting the multiple facets of a subsiding landscape from coastal cities and wetlands to the continental shelf

Land subsidence is a settling, sinking, or collapse of the land surface. In the southeastern United States, subsidence is frequently observed as sinkhole collapse in karst environments, wetland degradation and loss in coastal and other low-lying areas, and inundation of coastal urban communities. Human activities such as fluid extraction, mining, and overburden alteration can cause or exacerbate subsidence, which can result in damage to infrastructure and resources. Subsidence is a hazard that takes place throughout the United States; however, a systematic approach to recognize and develop informed responses to the drivers of subsidence has not yet been fully established. To address this problem, the U.S. Geological Survey (USGS) Southeast Region (SER) funded the gathering of a team of interdisciplinary USGS scientists to promote scientific collaboration. Southeast Region scientists welcomed scientists from other regions (see table 1.1 in Appendix 1) in September 2018 at the St. Petersburg Coastal and Marine Science Center (SPCMSC) in Florida for the first workshop of the Subsidence Flex Team (SFT) (see Appendix 2 for agenda). The SFT set out to review subsidence-related research and technology and develop a unifying framework for describing the processes and hazards associated with land subsidence. A more comprehensive understanding of subsidence hazards could help to inform regional vulnerability assessments that would prove invaluable to the public, community developers, policy makers, and resource managers in both inland and coastal states. The SFT analyzed USGS strengths and weaknesses to identify existing infrastructure and capabilities that could be leveraged to create a comprehensive and far-reaching subsidence-monitoring and mitigation program. Over the course of the 2-day workshop, interdisciplinary understandings of the processes and hazards related to subsidence were explored through individual presentations and group discussion. With all perspectives considered, the SFT recommended that subsidence-related research develop scientific approaches and metrics by which the subsidence component can be isolated and quantified in order to protect both the environment and human infrastructure from harm.

Alabama, Florida, Louisiana, Mississippi↗

USGS2021

Correspondence analysis for mineral commodity research: An example workflow for mineralized calderas, southwest United States

Historical mine and mineral deposit datasets are routinely used to inform quantitative mineral assessment models, but they also can contain a wealth of supplementary qualitative information that is generally underutilized. We present a workflow that uses correspondence analysis, an exploratory tool commonly applied to multivariate abundance data, to better utilize qualitative data in these historical datasets. The workflow involves extraction of qualitative information on ore mineralogy from a mineral deposit database, attaches those data to a target geological feature, and analyzes the underlying data structure with correspondence analysis and hierarchical clustering. The output of correspondence analysis is inversely weighted to the relative frequency of ore minerals, and therefore rare mineral species (i.e., those with unusually low frequencies) can disproportionately contribute to the total variance of the dataset. We present a novel technique for aggregating frequencies of rare mineral species that minimizes this effect. We apply this workflow to evaluate how ore mineral assemblages in former and active mines vary in spatial relation to silicic calderas in the southwestern United States. The most common ore mineral associations observed spatially and genetically associated to calderas include those related to polymetallic, base metal-rich systems and epithermal Au–Ag systems. Three other groups of mineralized calderas were identified, including: (1) Hg–Sb mineralized calderas in the northern Great Basin and western Nevada volcanic field; (2) calderas associated with elevated abundances of Mn oxides/hydroxides, fluorite, and Be-minerals, mostly in eastern Utah and New Mexico; and (3) calderas with numerous U ± F deposits, which are located in central Colorado, the eastern Great Basin and in northern Nevada. The latter three groups are associated with economically significant critical mineral resources, including the Li resources of the McDermitt complex and Be associated with the Spor Mountain on the margin of the Thomas caldera complex. We conclude that correspondence analysis is a promising technique that can enhance data exploration of the qualitative information held within mineral deposit datasets. Consequently, it could have numerous applications for mineral potential mapping, resource assessment projects, and characterization of mineral systems.

Natural Resources Research↗

USGS1976

Discovery of natural resources

Mankind will continue to need ores of more or less the types and grades used today to supply its needs for new mineral raw materials, at least until fusion or some other relatively cheap, inexhaustible energy source is developed. Most deposits being mined today were exposed at the surface or found by relatively simple geophysical or other prospecting techniques, but many of these will be depleted in the foreseeable future. The discovery of deeper or less obvious deposits to replace them will require the conjunction of science and technology to deduce the laws that governed the concentration of elements into ores and to detect and evaluate the evidence of their whereabouts. Great theoretical advances are being made to explain the origins of ore deposits and understand the general reasons for their localization. These advances have unquestionable value for exploration. Even a large deposit is, however, very small, and, with few exceptions, it was formed under conditions that have long since ceased to exist. The explorationist must suppress a great deal of "noise" to read and interpret correctly the "signals" that can define targets and guide the drilling required to find it. Is enough being done to ensure the long-term availability of mineral raw materials? The answer is probably no, in view of the expanding consumption and the difficulty of finding new deposits, but ingenuity, persistence, and continued development of new methods and tools to add to those already at hand should put off the day of "doing without" for many years. The possibility of resource exhaustion, especially in view of the long and increasing lead time needed to carry out basic field and laboratory studies in geology, geophysics, and geochemistry and to synthesize and analyze the information gained from them counsels against any letting down of our guard, however (17). Research and exploration by government, academia, and industry must be supported and encouraged; we cannot wait until an eleventh hour to mount a "crash program.".

Science↗

USGS1993

Summary of the mineral- and energy-resource endowment, BLM roswell resource area, east-central New Mexico

In this summary of two comprehensive resource reports produced by the U.S. Bureau of Mines and the U.S. Geological Survey for the U.S. Bureau of Land Management, we discuss the mineral- and energyresource endowment of the 14-millon-acre Roswell Resource Area, New Mexico, managed by the Bureau of Land Management. The Bureau and Survey reports result from separate studies that are compilations of published and unpublished data and integrate new findings on the geology, geochemistry, geophysics, mineral, industrial, and energy commodities, and resources for the seven-county area. The reports have been used by the Bureau of Land Management in preparation of the Roswell Resource Area Resource Management Plan, and will have future use in nationwide mineral- and energy-resource inventories and assessments, as reference and training documents, and as public-information tools. In the Roswell Resource Area, many metals, industrial mineral commodities, and energy resources are being, or have been, produced or prospected. These include metals and high-technology materials, such as copper, gold, silver, thorium, uranium and/or vanadium, rare-earth element minerals, iron, manganese, tungsten, lead, zinc, and molybdenum; industrial mineral resources, including barite, limestone/dolomite, caliche, clay, fluorspar, gypsum, scoria, aggregate, and sand and gravel; and fuels and associated resources, such as oil, gas, tar sand and heavy oil, coal, and gases associated with hydrocarbons. Other commodities that have yet to be identified in economic concentrations include potash, halite, polyhalite, anhydrite, sulfur, feldspar, building stone and decorative rock, brines, various gases associated with oil and gas exploration, and carbon dioxide. ?? 1993 Oxford University Press.

Nonrenewable Resources↗

USGS2010

Co-Cu-Au deposits in metasedimentary rocks-A preliminary report

A compilation of data on global Co-Cu-Au deposits in metasedimentary rocks refines previous descriptive models for their occurrence and provides important information for mineral resource assessments and exploration programs. This compilation forms the basis for a new classification of such deposits, which is speculative at this early stage of research. As defined herein, the Co-Cu-Au deposits contain 0.1 percent or more by weight of Co in ore or mineralized rock, comprising disseminated to semi-massive Co-bearing sulfide minerals with associated Fe- and Cu-bearing sulfides, and local gold, concentrated predominantly within rift-related, siliciclastic metasedimentary rocks of Proterozoic age. Some deposits have appreciable Ag ? Bi ? W ? Ni ? Y ? rare earth elements ? U. Deposit geometry includes stratabound and stratiform layers, lenses, and veins, and (or) discordant veins and breccias. The geometry of most deposits is controlled by stratigraphic layering, folds, axial-plane cleavage, shear zones, breccias, or faults. Ore minerals are mainly cobaltite, skutterudite, glaucodot, and chalcopyrite, with minor gold, arsenopyrite, pyrite, pyrrhotite, bismuthinite, and bismuth; some deposits have appreciable tetrahedrite, uraninite, monazite, allanite, xenotime, apatite, scheelite, or molybdenite. Magnetite can be abundant in breccias, veins, or stratabound lenses within ore or surrounding country rocks. Common gangue minerals include quartz, biotite, muscovite, K-feldspar, albite, chlorite, and scapolite; many deposits contain minor to major amounts of tourmaline. Altered wall rocks generally have abundant biotite or albite. Mesoproterozoic metasedimentary successions constitute the predominant geologic setting. Felsic and (or) mafic plutons are spatially associated with many deposits and at some localities may be contemporaneous with, and involved in, ore formation. Geoenvironmental data for the Blackbird mining district in central Idaho indicate that weathering of abundant Fe, S, As, Co, and Cu in sulfide minerals of the deposits produces acidic waters, especially in pyrite-rich deposits; mine runoff has high concentrations of Fe, Cu, and Mn that exceed U.S. drinking water or aquatic life standards.

Open-File Report↗

USGS1994

Lead isotope compositions as guides to early gold mineralization: The North Amethyst vein system, Creede district, Colorado

The North Amethyst vein system, which is hosted by approximately 27 Ma Carpenter Ridge Tuff and approximately 26 Ma Nelson Mountain Tuff, has two mineral associations separated by brecciation and sedimentation in the veins. The early association consists of quartz, rhodonite, hematite, magnetite, electrum (Au (sub 0.3-0.5) Ag (sub 0.7-0.5)) , and Mn carbonate, Au-Ag sulfide, Ag sulfosalt, and base metal sulfide minerals. The later mineral association cuts the Mn- and Au-bearing assemblages and consists of quartz, calcite, sericite, chlorite, hematite, adularia, fluorite, base metal sulfides, and Ag-bearing tetrahedrite.Our detailed studies show that the Pb isotope compositions of paragenetically early galenas associated with Au-rich mineralization in the North Amethyst vein system are relatively unradiogenic ( 206 Pb/ 204 Pb: 18.826-18.881, 207 Pb/ 204 Pb: 15.588-15.602, and 208 Pb/ 204 Pb: 37.790-37.926) compared to Pb isotope compositions of galenas formed later at about 25 Ma during Ag and base metal mineralization ( 206 Pb/ 204 Pb: 19.041-19.115, 207 Pb/ 204 Pb: 15.627- 15.672, and 208 Pb/ 204 Pb: 37.829-38.057). New Pb isotope data for the central and southern parts of the Creede district, which are located 5 to 7 km south of the North Amethyst area, agree with the results of a regional study by Doe et al. (1979) that included five samples from the main part of the Creede district. Galenas and adularia from the central and southern Creede district and galenas from the Bondholder district, 7 km north of the North Amethyst area, are similar to the Pb isotope compositions of galenas formed later in the North Amethyst area during Ag and base metal mineralization. Galenas from the Alpha-Corsair vein, which was mined for Ag prior to 1910, are isotopically similar to galenas associated with North Amethyst Au-stage mineralization. This isotopic similarity suggests that unexplored segments of the Alpha-Corsair structure may have the same mineralogy as the North Amethyst Au stage; thus, the Alpha-Corsair structure has the potential for high gold contents.Pb isotope compositions from the late stage of the North Amethyst vein system and from the Bondholder and central and southern Creede mining districts are more radiogenic than the host volcanic rocks of the central cluster of the San Juan volcanic field. Our Pb isotope results indicate that early Au mineralization of the North Amethyst area may represent the product of an older and relatively local hydrothermal system distinct from that of the younger base metal and Ag mineralization found throughout the region. Fluids that deposited Au minerals may have derived their Pb isotope composition by a greater degree of interaction with shallow, relatively less radiogenic volcanic wall rocks. The younger, base metal and Ag-rich mineralization that overprints the Au mineralization in the North Amethyst area clearly has a more radiogenic isotopic signature, which implies that the later mineralization derived a greater component of its Pb from Proterozoic source rocks, or sediments derived from them.Paragenetically early sulfide-rich vein assemblages have the least radiogenic galenas and generally also have the highest Au contents. Thus, identification of paragenetically early vein assemblages with relatively unradiogenic Pb isotope compositions similar to those of the North Amethyst area provides an additional exploration tool for Au in the central San Juan Mountains area.

Colorado↗

USGS1996

Application of a feedforward neural network in the search for kuroko deposits in the Hokuroku District, Japan

A feedforward neural network with one hidden layer and five neurons was trained to recognize the distance to kuroko mineral deposits. Average amounts per hole of pyrite, sericite, and gypsum plus anhydrite as measured by X-rays in 69 drillholes were used to train the net. Drillholes near and between the Fukazawa, Furutobe, and Shakanai mines were used. The training data were selected carefully to represent well-explored areas where some confidence of the distance to ore was assured. A logarithmic transform was applied to remove the skewness of distance and each variable was scaled and centered by subtracting the median and dividing by the interquartile range. The learning algorithm of annealing plus conjugate gradients was used to minimize the mean squared error of the scaled distance to ore. The trained network then was applied to all of the 152 drillholes that had measured gypsum, sericite, and pyrite. A contour plot of the neural net predicted distance to ore shows fairly wide areas of 1 km or less to ore; each of the known deposit groups is within the 1 km contour. The high and low distances on the margins of the contoured distance plot are in part the result of boundary effects of the contouring algorithm. For example, the short distances to ore predicted west of the Shakanai (Hanaoka) deposits are in basement. However, the short distances to ore predicted northeast of Furotobe, just off the figure, coincide with the location of the Nurukawa kuroko deposit and the Omaki deposit, south of the Shakanai-Hanaoka deposits, seems to be on an extension of short distance to ore contour, but is beyond the 3 km limit from drillholes. Also of interest are some areas only a few kilometers from the Fukazawa and Shakanai groups of deposits that are estimated to be many kilometers from ore, apparently reflecting the network's recognition of the extreme local variability of the geology near some deposits.

Mathematical Geology↗

USGS1985

xygen isotope, aeromagnetic, and gravity anomalies associated with hydrothermally altered zones in the Yankee Fork mining district, Custer County, Idaho

Epithermal Ag-Au vein and disseminated deposits in the Yankee Fork district are hosted in altered volcanic rocks having low delta 18 O values, low magnetic susceptibilities, low remanent magnetizations, and relatively high densities. These isotopic and physical quantities provide sensitive indices of rock alteration that can be contoured over areas that are much larger than zones of strong visible alteration. Such contour maps reveal a marked spatial association between the ore deposits, the steepest delta 18 O gradients, and the perimeter of zones where the rocks have extremely low total magnetizations. The oxygen isotope results indicate that the ore zones coincide with steep gradients in temperature and water/rock ratio in two fossil hydrothermal convective systems that involved fluids derived from Tertiary meteoric waters. Our magnetic measurements suggest that these same high-gradient zones also coincide with oxidation-reduction boundaries. Low aeromagnetic intensities and positive Bouguer anomalies are also associated with the altered zones in the Yankee Fork district. The intensities of these geophysical features are consistent with the size of the altered zones and with the contrast in physical properties between altered and unaltered rocks. Contour maps of the magnetic and isotopic properties of host rocks should find application in exploration for a variety of types of hydrothermal ore deposits, as well as in the study of ore controls and conditions of mineralization.

Economic Geology↗

USGS1985

Regional setting and new information on some critical geologic features of the West Shasta district, California

The West Shasta massive sulfide district is in the easternmost of a series of accreted island-arc and oceanic crust terranes that comprise the Klamath Mountains. A sequence of submarine volcanic rocks of predominantly Early Devonian age is the principal component of the island-arc terrane in which the sulfide deposits are hosted. The Copley Greenstone, consisting mainly of andesitic and basaltic pillow lavas and breccias totaling at least 1,800 m in thickness, is the oldest rock unit in the sequence. It is overlain and also intruded by dikes of the Balaklala Rhyolite. Northeast of the West Shasta district, greenstone also overlies the Balaklala Rhyolite, suggesting that a major greenstone unit may overlie potentially mineralized rhyolite east of the district. However, recent studies have shown that the complex relations in that area can be explained by folding.The Balaklala sequence consists of silicic flows, breccias, and tuffs having a maximum thickness somewhat in excess of 1,000 m. The Balaklala has been divided by Kinkel et al. (1956) into three units: a lower nonporphyritic to slightly porphyritic unit containing large amounts of breccia and tuff; a middle unit characterized by rhyolite containing quartz phenocrysts 1 to 4 mm in diameter but also containing a complex assortment of tuff, breccia, and pyritic massive sulfide bodies in its upper part; and an upper unit typically containing dark quartz phenocrysts in excess of 4 mm in diameter. Much of the lower part of the upper unit is pyroclastic material, whereas most of the upper part of the unit appears to be a massive volcanic-flow rock. At least half a dozen eruptive centers for the Balaklala Rhyolite are identified, three of them within an area measuring 17 X 3 km that constitutes the limits of the former mining district.The youngest unit in the arc sequence is the Kennett Formation consisting of black shale and chert containing radiolarians, which indicate fairly deep-water deposition in its lower part, and limestone containing a shallow water fauna of probable Middle Devonian age in the upper part. A 400-m.y.-old trondhjemite stock, named the Mule Mountain stock, intrudes the Copley Greenstone and Balaklala Rhyolite and is considered to be essentially coeval with these volcanic units.The pyritic massive sulfide deposits occur in clusters of individual bodies owing in large part to disruption by postmineral faults. The deposits are stratigraphically confined to the upper part of the subhorizontal middle unit of the Balaklala, and their horizontal distribution is here interpreted to have been controlled by an extensional tectonic regime that prevailed during Early Devonian time. The major geologic evidence for such a regime is the marked preferred elongations exhibited by the distribution of the afore-mentioned eruptive centers, the eight or nine massive sulfide clusters in the district, and the geometrically similar distribution of the clusters in relation to each other. The preferred directions of elongation in both the detailed and broader senses may be generalized to N 20 degrees to 25 degrees E, N 37 degrees E, N 60 degrees to 80 degrees E, and N 40 degrees to 60 degrees W. The three northeast trends dominate throughout the district, whereas the northwest trend is evident mainly in the northern part and is more speculative. The major deposit clusters fall at intersections of the major trends, some of which may have been grabens. Trend intersections having no known deposits may be good exploration targets in localities where the stratigraphically favorable middle unit of the Balaklala is still present.

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